The present invention is related to vehicle wheel balancer systems having a spindle shaft upon which a vehicle wheel assembly undergoing an imbalance measurement and correction procedure is mounted, and in particular, to a method for detecting and identifying deviations of the actual rotational position of the vehicle wheel assembly mounted on the spindle shaft from an expected rotational position of the vehicle wheel assembly after an imbalance measurement.
Vehicle wheel assemblies, consisting of a tire mounted to a wheel rim, generally have at least a small amount of non-uniformity about an axis of rotation which is experienced as vibration when the wheel assembly is installed on a vehicle and driven at road speeds. These non-uniformities may be the result of manufacturing defects in either the rim or tire, damage to the rim or tire, or spring-rate variations about the circumference of the tire. A spring rate variation relates to a tire's stiffness at various points about the circumference of the tire. As a stiff point rotates into contact with a road surface, the tire reacts differently when compared with the reaction resulting from the contact of a softer point with the road surface. This produces a measurable force variation at a rotational speed of the tire, leading to a first harmonic vibration.
Vehicle wheel balancer systems, such as shown in FIG. 1, are configured to measure the imbalance (static and dynamic) of a vehicle wheel assembly by mounting the vehicle wheel assembly on a spindle shaft, rotationally driving the spindle shaft and mounted vehicle wheel assembly, and measuring the resulting vibrations or forces through suitably placed transducer elements. By processing the signals from the transducer elements, together with rotational position signals associated with the spindle shaft, a balancer processing system can identify the imbalance present in the vehicle wheel assembly, and direct an operator to apply imbalance correction weights at specific rotational and axial positions about the vehicle wheel assembly. In order to assist the operator in this task, a vehicle wheel balancer system may be configured to rotationally drive the vehicle wheel assembly to position the application site for an imbalance correction weight at a predetermined rotational position.
In order to measure force variations in a vehicle wheel assembly mounted to the spindle shaft of a vehicle wheel balancer, the vehicle wheel balancer system must be equipped with additional instrumentation. For example, as shown in U.S. Pat. Nos. 6,422,074, 6,435,027, 6,405,591, and 6,336,364, each of which is herein incorporated by reference, an instrumented drum or load roller is movable to engage with an outer circumferential surface of the inflated tire as the vehicle wheel assembly is rotationally driven on the spindle shaft. Signals representative of the forces exerted on the instrumented drum or roller by the rotating vehicle wheel are processed by the balancer processing system, together with the rotational position signals associated with the spindle shaft, to identify the spring rate variations about the circumference of the tire.
Each of these measurement procedures requires that the vehicle wheel assembly remain in a fixed position relative to the spindle shaft upon which it is mounted, such that sensors monitoring the rotation of the spindle shaft can accurately identify the corresponding rotational position of the vehicle wheel assembly. Rotational mounting slippage of the vehicle wheel assembly about the rotational axis of the spindle shaft during or after a measurement procedure is generally difficult to detect, and may result in the vehicle wheel balancer processing system directing an operator to apply an imbalance correction weight at an incorrect rotational position on the vehicle wheel assembly, as the wheel assembly is not rotationally positioned where expected relative to the balancer spindle shaft. This slippage may result from an operator's failure to properly secure the vehicle wheel assembly to the spindle, or in response to a sudden acceleration or deceleration of the vehicle wheel assembly during rotation on the spindle shaft, such as may occur during engagement of the vehicle wheel assembly with an instrumented drum or roller.
A similar, albeit rare, source of error may be introduced if the spindle shaft itself rotationally slips relative to a rotational position sensor of the vehicle wheel balancer system which is monitoring the shaft rotation. Under these circumstances, the vehicle wheel balancer processing system will again direct the operator to apply an imbalance correction weight at an incorrect rotational position of the vehicle wheel assembly, as the processing system is unable to detect the deviation of the spindle shaft actual rotational position from the rotational position reported by the position sensor.
Application of an imbalance correction weight at an incorrect rotational position on the vehicle wheel assembly will fail to correct the measured imbalance of the vehicle wheel assembly, which will result in the vehicle wheel assembly failing a follow-up check spin. While current vehicle wheel balancer systems can identify when an imbalance of the vehicle wheel assembly has not been fully corrected by the application of the imbalance correction weights, they cannot identify the source of the error, and can only direct the operator to repeat the imbalance measurement procedures to alter the amount and placement of the imbalance correction weights. If the wheel assembly continues to slip, the operator may become caught in a repeating cycle of failed measurements, commonly referred to as “chasing weights”.
Accordingly, it would be advantageous to provide a vehicle wheel balancer system with a method to evaluate a remaining imbalance present in a vehicle wheel assembly following the application of imbalance correction weights, and to determine if any remaining imbalance is the result of the vehicle wheel assembly having deviated from an expected rotational position during the time between the measurement of the imbalance and the application of the imbalance correction weights.